Such a method and such an apparatus are known from WO 00/44427 A1. WO 00/44427 A1 deals with the problem of the artificial ventilation of patients with an ailing lung. The basic patho-physiological mechanism of an ailing lung is the lack of surfactant (substance which reduces surface tension) which can cause a collapse of major lung fractions and a dramatically reduced gas exchange area. Hence, to prevent undesirable sequelae and consecutive multiorgan failure, an important goal of protective ventilator therapy is a gentle and early “reopening” of the lung. Through the identification of the alveolar opening and especially of the alveolar closing pressures, a distressed lung may be kept open by proper choice of the airway pressure. However, the manual determination of opening and closing pressures is arduous and time consuming. Therefore, WO 00/44427 A1 suggests to use the partial pressures of oxygen (paO2) as an indicator for determining the opening and closing pressures of the lung. WO 00/44427 A1 has recognized that there is a significant hysteresis behaviour of the paO2 as a function of the ventilation pressure.
FIG. 1 shows the paO2 hysteresis of the same healthy (left) and ailing (right) lung. While there is almost no hysteresis in the healthy lung and the choice of ventilation pressures has no visible impact on the quality of gas exchange, the hysteresis is even more severe in an ailing lung. In many cases, gas exchange may be reduced so strongly that at typical ventilation pressures, a sufficient hemoglobin oxygen saturation (>85 mm Hg) may only be reached if high oxygen concentrations (e.g. 90 . . . 100%) are delivered to the patient.
For such an ailing lung, a ventilation strategy could be to first open the lung with a temporary high inspiratory airway pressure and then ventilate on the descending branch of the hysteresis such that a sufficient tidal volume is reached and gas exchange is maintained. This so called recruitment maneuver has become a common strategy in operating rooms and in the intensive care medicine. In general, to achieve a sufficient tidal volume it is necessary to ventilate the lung with a certain delta pressure, which is defined as:delta pressure=PIP−PEEPPIP is the peak inspiratory pressure and PEEP is the positive end expiratory pressure. The aim of the recruitment maneuver is to find the alveolar opening pressure and the alveolar closing pressure. It is then possible to set the peak inspiratory pressure slightly higher than the alveolar opening pressure and to set the positive end expiratory pressure slightly higher than the alveolar closing pressure. In this way ideally all previously closed lung units will be re-opened and at the same time all open lung units will be kept open.
During a recruitment maneuver the peak inspiratory pressure is stepwise increased so that as many lung units as possible are re-opened, while at the same time the positive end expiratory pressure is increased in order to keep the newly recruited lung units open. When recruiting a lung, some lung units open up and become overdistended, while other lung units are still closed. Thus, when increasing the peak inspiratory pressure in order to re-open as many lung units as possible, most of the opened lung units will be overdistended.
Due to the hysteresis behaviour of the lung, the values obtained for peak inspiratory pressure and for the positive end expiratory pressure during this process of a stepwise increase are too high to further ventilate the lung once the lung units are opened. Thus they need to be reduced systematically.
At first the excessive peak inspiratory pressure is reduced while the positive end expiratory pressure is maintained at its level. This reduction is performed until an adequate tidal volume is reached. From this point onwards both the peak inspiratory pressure and the positive end expiratory pressure are reduced simultaneously. The aim is to find the lowest value for the positive end expiratory pressure that would just maintain all re-opened lung units open. At this stage the peak inspiratory pressure is a secondary variable of interest. Noticeably, the tidal ventilation will change during this simultaneous reduction of the peak inspiratory pressure and the positive end expiratory pressure, since the relief of overdistension will initially increase the lung's compliance. Once the positive end expiratory pressure is too low to keep all previously re-opened lung units open, the point of alveolar closing is reached.
Having identified the values of the peak inspiratory pressure corresponding to the alveolar opening pressure and the positive end expiratory pressure corresponding to the alveolar closing pressure as outlined above, it is then possible to ventilate the lung in an optimal condition. First, all lung units are re-opened by choosing a peak inspiratory pressure which is slightly higher than the alveolar opening pressure, i.e. 2-5 cmH2O higher, and choosing a positive end expiratory pressure which is slightly higher than the alveolar closing pressure, i.e. 2-3 cmH2O higher. Afterwards the peak inspiratory pressure is reduced again to achieve the desired tidal volume. The corresponding ventilation stage corresponds to the optimal condition. An optimal compliance is achieved, since all lung units are opened, and no major overdistension is present.
By way of an example, FIG. 2 shows a typical recruitment maneuver in detail. As shown in FIG. 2, the recruitment maneuver is carried out on the basis of a pressure controlled ventilation. Before the final recruitment maneuver takes place, the alveolar opening pressure and the alveolar closing pressure have to be identified. In a first step (step 1), PIP and PEEP are stepwise increased by means of an incremental limb until the alveolar opening pressures have been detected with regard to PIP and PEEP (steps 2 and 3). The alveolar opening pressure with regard to PIP is usually about 40 cmH2O in normal lungs and in the range of 55-60 cmH2O in sick lungs. After a successful alveolar opening, a decremental limb or stepwise decrease of PIP and PEEP is done (step 4) to determine the alveolar closing pressure (step 5). As outlined above initially only PIP is reduced as indicated at the transition from step 3 to step 4 in FIG. 2. After having identified the pressures for alveolar opening and alveolar closing, the final recruitment maneuver (step 6) is done with these new target pressures over 10 breaths and PEEP is set above the alveolar closing pressure to avoid pulmonary re-collapse. For example, PEEP is set 2 cmH2O above the alveolar closing pressure, i.e.PEEP=PEEPclose+2 cmH2O
Alternatively, a volume controlled ventilation can be carried out having the advantage that the ventilated volume remains constant and that all changes of the lung status can be related to changes within the alveoli.
In order to avoid the invasive measurement of paO2, WO 00/44427 A1 utilizes according to a first embodiment the endtidal CO2 concentration (etCO2) and/or the CO2 output as feedback signals for identification of the optimal ventilator settings for ailing lungs. Both feedback signals can be measured non-invasively. etCO2 can be obtained by measuring the CO2 concentration at the end of an expiration cycle. CO2 output (unit [ml CO2/min]) can be obtained from continuous measurements of the CO2 concentration (unit [%]) and air flow (unit [ml/min]) and subsequent breathwise computation of
            V      .                      CO        2            ⁢      Atom        =      RR    ·                  ∫        0        T            ⁢                        [                      CO            2                    ]                ⁢                              (            t            )                    ·                                                    V                .                            Atom                        ⁡                          (              t              )                                      ⁢        𝔻        ⁢                                  ⁢        T            during one expiration cycle. According to a second embodiment of WO 00/44427 A1, the hemoglobin oxygen saturation (SO2) is measured non-invasively and is used as a feedback signal for identification of optimal ventilation parameters for ailing lungs.
In summary, WO 00/44427 A1 discloses a non-invasive method for determining the alveolar opening or closing of a lung based on one of the measurement of the parameters CO2 concentration (etCO2), CO2 output or hemoglobin oxygen saturation (SO2). However, practical tests have shown various disadvantages of this method. One disadvantage is the fact that a single parameter is subject of various disturbances so that an average value of several parameters has to be taken over several breath cycles which causes a delay in the feed back path. Another disadvantage is the fact that the detection of alveolar opening cannot be clearly distinguished from an overdistension of the lung which could cause severe damages to the lung during the recruitment maneuver.